Conventional mechanical grippers use several “fingers” that can be either rigid or non-rigid and that retract/close upon an object to form a grasp. The grasp geometry is usually predetermined, and in most cases grippers can only grasp specific objects by forming contact with specific points on it. Grippers that are meant to be versatile usually rely on redundancy (a large number of fingers), or soft, flexible fingers. Versatile grippers often lack the ability to withstand large external wrenches applied to the grasped object due to their flexible nature. Rigid grippers specified for specific object manipulation (such as those in an industrial assembly line) are good at withstanding external wrenches applied on the grasped object but in many cases cannot grasp even a slightly different object without corresponding hardware and/or software adaptation.
Fingertips of grippers are traditionally rigid, made of plastic, metal, hard rubber etc. The fingertips are designed to contact the object in a manner that does not harm it, while applying direct force and frictional force that suffice to manipulate the object and withstand external wrenches applied to it. The contact points are predetermined and often even predesigned to fit the fingertip for improved grasping.
While existing grippers are good at manipulating predetermined objects, there is a need for gripping devices that can grasp differently shaped objects without prior knowledge of their geometry, and manipulate them. The present invention is directed toward providing such a solution.
“Blind” closing of gripper fingers onto an object without predetermination of the contact locations can result in either:
A. An immobilizing grasp;
B. “Jamming” (an unstable, frictional grasp);
C. The object will escape and not be grasped.
If the fingers are retracted from what is known as a “caging” position, option C will not occur regardless of the finger type. Frictionless fingertips will result (in most cases) in option A. The fingers will close onto the object, which in turn will move within the retracting cage, until there is no further possible movement and the volume of the cage is zero, constituting an immobilizing grasp. Frictional fingertips will result (in most cases) in option B. The frictional contacts are likely to create a frictional grasp within the cage with positive non-zero volume, leaving room for the object to move if dislodged from the frictional contacts given a sufficient external wrench. Both A and B are considered feasible grasps, however they may be susceptible to external wrenches applied on the object, resulting in a loss of fingertip-object contact, or a “sliding” of the object along the fingertips. In case A this could occur due to the natural compliance and flexibility of the robotic gripper and of the object, and the lack of frictional forces to resist a sliding motion. In case B this could occur due to external wrenches dislodging the frictional contacts from their grasping positions, resulting in a freely-moving object (within the cage). A combination of an immobilizing grasp that is also a frictional grasp is the most robust to such external wrenches, but is difficult to obtain without engineering the contact in advance. Indeed, most manipulation applications utilize such grasps by exactly placing frictional fingertips in predetermined positions on the object that form an immobilizing grasp.
US2017/0036354 relates to a two-phase gripper. The gripper reorients and grasps an object while being picked up. The gripper includes a parallel jaw gripper including a pair of opposed, two-phase fingers, each finger including a cavity covered by an elastic strip wherein the elastic strip includes a point contact. Closure of the jaws of the gripper on as object at a first relatively lower force results in contact with lower friction between the point contact on the elastic strip on the fingers and the object allowing the object to rotate under gravity as the gripper is raised. Thereafter, closure of the jaws of the gripper on the object at a second relatively higher force causes the elastic strip to receded into the cavity resulting in multi-point contact with higher friction between the fingers and the object to securely grasp the object.
It is therefore an object of the present invention to provide a method and means for a robust versatile grasping of a variety of differently shaped objects.
It is a further object of the present invention to provide a method and means for a robust versatile grasping with minimal or no physical adaptation required to the manipulated objects.
Other objects and advantages of the present invention will become apparent as the description proceeds.